7 STRUCTURE OF RNA  RNA or ribonucleic acid is present in all the living cells. It is laevo rotatory and is responsible for learning and memory. It is found in the cytoplasm as well as nucleus. Description of types and structure of RNA is given below. Types of RNA  In bacteria, there are three major types of RNAs: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).  All three RNAs are needed to synthesise a protein in a cell.  The mRNA provides the template, tRNA brings aminoacids and reads the genetic code, and rRNAs play structural and catalytic role during translation.  RNA is generally involve in protein synthesis but in majority of plant viruses, it serves as a genetic material. Therefore, there are two major types of RNA-(A) genetic RNA and (B) non-genetic RNA. (A) Genetic RNA Fraenkel-Conrat showed that RNA present in TMV (Tobacco Mosaic Virus) is a genetic material. Since then, it is established that RNA acts as a genetic material in most plant viruses. (B) Non-genetic RNA. This type of RNA is commonly present in cells where DNA is genetic material. Non-genetic RNA is synthesized on DNA template. It is of following three major types: (a) Messenger RNA (mRNA).  It was reported by Jacob and Monad.  It carries genetic information present in DNA, so is called working copy.  mRNA constitutes about 3-5% of the total RNA present in the cell.  The molecular weight varies from 25,000 to 1,00,000.  It is about 300 nucleotide long at minimum.  Structure of prokaryotic m-RNA. It is polycistronic in nature i.e., several cistrons (functional part of DNA) form a single m-RNA. Thus, each m-RNA has a message to produce several polypeptides. Average life is 2 minutes.  At 5' end near initiation codon a sequence of fixed bases is found called Shine Dalgarno (SD) sequence (5'AGGAGGU3'). It helps in correct binding of ribosomal subunit (30 S) on it.  Structure of eukaryotic m-RNA. It is monocistronic in nature and has a message to produce on polypeptide only. They are metabolically stable and their life varies from few hours to days. Fig: Eukaryotic mRNA contains introns that must be spliced out. A 5' cap and 3' poly-A tail are also added.  UTR (Untranslated region). They are sequences of RNA before start or initiation codon and after stop or termination codon. They are not translated. They are transcribed as part of same transcript as the coding region. Such UTRs provide stability (b) to m-RNA and also increase translational efficiency. Ribosomal RNA (rRNA). It was reported by Kuntz. It is most stable type of RNA and is a constituent of ribosomes. It forms about 80% of the total cellular RNA. In eukaryotes, 4 types of rRNA's found are 28 S, 18 S, 5.85 S and 5 S whereas, in prokaryotes three types of rRNA's found are 23 S, 16 S and 5 S. It is synthesised by genes present on DNA of several chromosomes found within a region known as nucleolar organiser. Fig: Ribosomal RNA (rRNA) Fig: Ribosome Structure Each ribosome consists of a large and a small subunit. The subunits remain separate when they are not in use for protein synthesis. (c) Transfer RNA (tRNA). The existence of tRNA was postulated by Crick. It is also known as soluble RNA (sRNA). These are the smallest molecules which carry amino acids to the site of protein synthesis. These constitute about 15% of the total cellular RNA. It acts as an intermediate molecule between triplet code of mRNA and amino acid sequence of polypeptide chain. All tRNA's have almost same basic structure. There are over 60 types of tRNA. Structure of t-RNA  Clover leaf model of tRNA is a 2-dimensional model suggested by Holley et.al. tRNA molecule appears like a clover leaf, being folded with three or more double helical regions (stem), having loops also. The three dimensional structure of the tRNA was proposed to be inverted L-shaped by Kim and Klug.  All tRNA molecules commonly have a guanine residue at its 5' terminal end. At its 3' end, unpaired – CCA sequence is present. Amino acid gets attached at this end only. The number of nucleotide varies from 77 (tRNAalanine) to 207 (tRNAtyrosine). There are three loops in t-RNA: (i) Amino acyl synthetase binding loop, also called DHU loop. (ii) Ribosomal binding loop with 7 unpaired bases. It is also called T-C loop. (iii) Anticodon loop with 7 unpaired bases. Out of the 7 bases in anticodon loop, 3 bases acts as anticodon for a particular triplet codon present on mRNA. Fig: Structure of tRNA Fig: Transfer RNA The stem and loop structure of a tRNA molecule is well suited to its functions: binding to amino acids, associating with mRNA molecules, and interacting with ribosomes. Differences between DNA and RNA FUNCTIONS OF RNA DNA serves two essential functions that deal with cellular information. First, DNA is the genetic material responsible for inheritance and is passed from parent to offspring for all life on earth. To preserve the integrity of this genetic information, DNA must be replicated with great accuracy, with minimal errors that introduce changes to the DNA sequence. A genome contains the full complement of DNA within a cell and is organized into smaller, discrete units called genes that are arranged on chromosomes and plasmids. The second function of DNA is to direct and regulate the construction of the proteins necessary to a cell for growth and reproduction in a particular cellular environment. In 1961, French scientists François Jacob and Jacques Monod hypothesized the existence of an intermediary between DNA and its protein products, which they called messenger RNA.1 Evidence supporting their hypothesis was gathered soon afterwards showing that information from DNA is transmitted to the ribosome for protein synthesis using RNA. There are three main types of RNA directly involved in protein synthesis are messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). If DNA serves as the complete library of cellular information, mRNA serves as a photocopy of specific information needed at a particular point in time that serves as the instructions to make a protein. The other types, rRNA and tRNA serve to aid the decoding of mRNA into the correct amino acid sequence which then folds into a protein. Proteins within a cell have many functions, including building cellular structures and serving as enzyme catalysts for cellular chemical reactions that give cells their specific characteristics. A gene is composed of DNA that is “read” or transcribed to produce an RNA (any type of RNA) molecule during the process of transcription. The processes of transcription and translation are collectively referred to as gene expression. Gene expression is the synthesis of a specific protein with a sequence of amino acids that is encoded in the gene. The flow of genetic information from DNA to RNA to protein is described by the central dogma. This central dogma of molecular biology further elucidates the mechanism behind Beadle and Tatum’s “one gene-one enzyme” hypothesis.